The present invention relates to methods and apparatus for forming a fused optical coupling between fibers Such coupling is generally accomplished by removing the jacket from a portion of each of two fibers, mechanically contacting, twisting, or otherwise preloading the bare cladding of the fibers against each other, and then applying heat to fuse the fibers while they are simultaneously drawn and elongated. The drawing of tapers out of the fibers brings the cores into sufficient proximity to couple the evanescent wave from one core to the other.
While such a coupler may be fabricated quickly and does not require precisely worked optical mounting components, the process does require attention to accurately align and to preload the fibers in contact, and the fusion/drawing process itself can seriously degrade fibers, particularly polarizing or polarization-maintaining fibers which rely on internal stresses to propagate separate polarization modes in the core. In particular, even assuming that the internal stress structure of a fiber remains undisturbed, for such fibers, the crosstalk between polarization modes (in Decibels) has been found to vary with 10 log tan.sup.2 .DELTA..theta., where .DELTA..theta. is the rotational misalignment in radians between principal axes of two fibers. Alignment to within a fraction of a degree is therefore necessary to prevent cross-coupling between one polarization eigenmode and the other. These internally asymmetric fibers have a nominally circular outer profile, which further complicates the tasks of maintaining them in rotational alignment with respect to each other, achieving a smooth taper of low slope, preserving geometrical symmetry and minimizing physical perturbations of the fiber.